You're set up

Click the icon in your toolbar any time to toggle dark mode for the current site, or the whole browser.

Tune it — brightness, contrast, sepia warmth and grayscale sliders let you match your eyes/monitor. Changes in the popup apply per-site so you can keep one page different from the rest.

Schedule it — open full settings and turn on a day/night schedule so dark mode switches on automatically at night.

Per-site control — already-dark sites (or ones that break) can be excluded individually without turning the extension off everywhere.